In one painting method which is frequently used for commercial operations, the atomized paint is electrostatically charged to a high voltage relative to the workpiece being coated. The electrostatic charge causes the atomized paint to be drawn to the workpiece. This significantly increases the paint transfer efficiency, thus reducing both the labor and material costs to paint the workpiece and reducing environmental problems. In a normal commercial painting operation, the workpiece is placed in a spray booth prior to painting. A low voltage power source, a compressed air hose and a paint hose are connected from exterior to the spray booth to the spray gun. The spray gun typically includes an oscillator and a voltage multiplying circuit for increasing the low voltage to a very high dc voltage for charging the paint as it is atomized. The high voltage may be, for example, up to 100 KV or more.
In some applications, it is not possible to use a conventional spray booth because of the size of the workpiece being sprayed. For example, an airplane will not fit into a conventional spray booth. In this case, the airplane hanger becomes the spray booth for confining the paint overspray and fumes. Since the paint may include flammable volatile organic compounds (VOC""s) as solvent, the hanger is considered a hazardous location and extreme care must be taken to prevent sparks which possibly could ignite the VOC vapors. A conventional low voltage power source which is connected to an electrostatic spray gun does not meet certain safety standards for operation in hazardous locations. Conventional switches, potentiometers and other circuit components and connections present a risk of sparks.
One method for meeting the hazardous location safety standards has been to place the low voltage source directly in the spray gun. As shown in U.S. Pat. Nos. 4,219,865 and 4,290,091, the spray gun may be provided with an internal generator which is driven by an air turbine for generating a low voltage without any electrical connections to the spray gun. Conventional voltage multiplying and rectifying circuitry is used in the spray gun to convert the low voltage to a high dc voltage for charging the paint. In operation, the turbine shares air with the atomization air delivered to the spray gun. The turbine air must be exhausted from the spray gun after use. In some applications, this can cause problems with dust agitation. The internal turbine and generator also significantly increase the weight of the spray gun which the operator must hold during spraying. Since aircraft are very large, the painting time may be quite long and the added weight can quickly tire the operator. Because of the rotary motion of the turbine in the spray gun, vibrations are transmitted into the handle of the spray gun. These vibrations are felt by the operator when the gun is used. The cost of a spray gun with an internal turbine and generator is quite high. Consequently, it is quite expensive to maintain spare spray guns, since each gun must include a turbine and a generator.
According to the invention, a low voltage power source including an electric generator and low voltage power regulating and control circuitry are mounted in a sealed, explosion proof housing which meets hazardous location safety standards. The low voltage power source is suitable for locating in a spray booth and in a hanger in which an airplane is being painted with paint having flammable solvents. The generator has a drive shaft which extends through and projects from the housing. An air driven turbine is attached to the housing and is connected to rotate the projecting turbine shaft. The low voltage power circuitry includes a voltage regulator and a potentiometer. The potentiometer has a shaft which extends through the housing for manually setting the output voltage level. The flamepath and the flame gap at the generator and potentiometer shafts are made to meet explosion proof standards. The turbine does not share the air source with the spray gun. The power source is located in an area where the exhaust air will not have an effect on the spray area. Since the rotating turbine and generator are not located in the spray gun, no vibrations are transmitted to the hand of the spray gun operator. Further, the spray gun will be much lighter than spray guns having an internal air turbine and generator and it will be less expensive for the user to maintain spare guns, since it is not necessary to purchase a turbine and a generator with each gun.
Accordingly, it is an object of the invention to provide an improved remote power generator for electrostatic paint applications which meets safety requirements for operation in hazardous locations.
Other objects and advantages of the invention will become apparent from the following detailed description of the invention and the accompanying drawings.